1. Field of the Invention
The present invention relates to an improved array antenna, and also relates to an antenna device including the improved array antenna and an antenna system employing such an antenna device.
2. Description of the Related Art
Japanese Patent Laid-Open Publication No. HEI-5-251928 discloses an antenna device, which includes an IC board with a transmitting and receiving circuit mounted thereon and a horn-type primary radiator. Further, Japanese Patent Laid-Open Publication No. HEI-8-97620 discloses an array antenna which includes a plurality of patch antennas arrayed on a dielectric substrate, a feed and feed lines or feeders connecting between the feed and the individual patch antennas. The feeders, in the form of microstrips, are formed on the dielectric substrate together with the patch antennas.
Among various known examples of the array antenna is a phased array antenna, which is designed to vary a phase difference between adjacent antenna elements to change the direction of radiated beams, to thereby switch the direction of the main lobe. The array antennas, which comprise an array of planar antennas of same structure, can be used as a phased array antenna by just varying a phase difference between adjacent antenna elements; however, it is difficult to vary their directivity of the array antenna depending on, for example, the size and distance (from the antenna) of objects that are to be detected. Further, the array antennas, having patches of individual planar antenna elements and associated feeders formed on a same planar surface, present the problem that their directivities would considerably deteriorate due to unwanted radiation of electromagnetic waves from the feeders, although they provide a very simple feeding scheme.